"We are still gathering information on this incident, but we repeat our warning that if the Assad regime uses chemical weapons, the United States and our allies will respond quickly and appropriately", State Department spokeswoman Morgan Ortagus said in a statement.
They are a major prize in fighting that has flared up in northwestern Syria in recent weeks between pro-government forces and the jihadists they have been used as a launchpad for rocket and drone attacks on the Hmeimim air base of the government's main ally Russian Federation.
The Trump administration ordered retaliatory strikes against Syrian regime targets in 2017 and 2018 after reports of chemical weapons attacks by Assad's forces.
"These conflicts, back and forth exchanges. just put tremendous pressure on civilians, they raise the specter of nation-to-nation clash", he said.
A woman salvages items from a building that was reportedly destroyed during air strikes by the Syrian regime ally Russian Federation, in the town of Kafranbel in the rebel-held part of the Syrian Idlib province on May 20, 2019.
Some 180 civilians have been killed in the flare-up since April 30, according to the Syrian Observatory for Human Rights, a British-based monitor.
"There were no civilians in the area", Abdel Rahman said.
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The Trump administration, alongside the United Kingdom and France, hit sites linked to Syria's chemical weapons production in response to chemical attacks in Idlib in 2017 and the Damascus suburb of Douma in 2018.
The US on Tuesday threatened reprisals on the Syrian government after signs that it had carried out a chemical attack during its offensive in Idlib.
"We can no longer put up with living under bombardment or in the open under the trees", said Abu Abdullah, one of thousands of Syrians in white tents dotted around the rock-strewn olive groves close to the frontier.
Troops loyal to President Bashar al-Assad seized control of Kafr Nabuda in Hama province on May 8 but were repelled by rebel fighters on Wednesday.
"There are a large number. killed from Assad's forces. there are many bodies still on the ground in the town", said Naji Mustafa, a spokesman for the Turkey-backed National Liberation Front.
The U.N.'s emergency relief coordinator, Mark Lowcock, said a full military offensive by the regime could result in the worst humanitarian catastrophe of the 21st century. Rebels said they had not documented the attack because they were under bombardment when it occurred.